Abstract

Abstract

Stable C isotope ratios (13C/12C) and amounts of soil carbonate were measured at six sites of differing temperature, moisture, and vegetational make-up in the northern Great Plains of the USA in order to determine climatic and biotic influences on the stable C isotope chemistry of these soils. With approximately constant precipitation, total carbonate decreased with increasing temperature. With decreasing precipitation, the total carbonate in the soils increased. For all but one soil, the 13C/12C ratios of segregated carbonate conformed to values expected if the carbonate formed from CO2 derived from soil organic matter. In the nonconforming site, the 13C/12C ratios of the carbonate suggest that the vegetation and soil organic matter have recently become dominated by C4 plants, which use the Hatch-Slack photosynthetic pathway, and organic matter derived from them. The 13C/12C ratios of disseminated carbonate reflect the proportion of pedogenic and parent-material carbonate in the sample. The 13C/12C ratios of pure, pedogenic carbonate at each site could be constrained by: (i) measured 13C/12C ratios of soil organic matter, (ii) measured 13C/12C ratios of carbonate nodules, and (iii) calculated 13C/12C ratios using a diffusion model designed by others. Estimates of the percentages of disseminated pedogenic carbonate in the soils suggested that, as the amount of weathering at sites increases and the total carbonate decreases, the percentage of carbonate that remains becomes more dominated by carbonate of pedogenic origin.

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